A technique, so called as a dye thermal transfer method, is known, in which an ink sheet containing a thermal diffusive colorant capable of thermally transferring by diffusion is faced to an image receiving layer of an image receiving sheet and the thermal diffusive colorant is imagewise transferred to the image receiving layer by a thermal printing means such as a thermal head or a laser to form an image. The thermal transfer method has been acknowledged as a method which enables forming an image using digital data without using any treatment liquid such as a developer, and forming an image having high quality comparable to that of a silver salt photograph.
It has been recognized that it is important to give a heat insulation function and a cushion function to the thermal image receiving sheet for obtaining a high quality image forming property in the recording system using the dye thermal transfer method.
For such problems, known is a method in which a foamed film having a function of heat insulation and a function of cushion is pasted on a substrate and an image receiving layer is provided on the foamed film. In this method, however, the foamed film shrinks by heat generated when the image receiving layer is applied on the foamed film, resulting in curling of the produced sheet. In order to overcome the problem, various methods have been tested, for example: (i) using a novel functional layer having a heat insulating function and a cushioning function; (ii) using a thermal transfer image receiving sheet produced by a process including no pasting step of the foamed film to minimize the curl caused by the heat generated in the production process; and (iii) using a variety of coating method.
For example, a thermal transfer image receiving sheet is disclosed, in which a resin containing aqueous liquid is foamed by mechanical stirring and coated to form a porous resin layer so that the apparent density of the porous layer is controlled to 0.05 to 0.5 g/cm3 (for example, Patent Document 1). However, in the above proposed method, it is not fully easy to stably obtain a desired foaming ratio of the coating liquid in the coating step of the industrial production process. Further, the coagulation of foam may not be controlled by the disclosed method, resulting in causing uneven image quality due to white spots in the image or lowering in the sensitivity.
Furthermore, a thermal transfer image receiving sheet is disclosed, in which the viscosity of a coating liquid containing a foamed particle having the heat insulation and the cushion functions is controlled into a certain range and an intermediate layer is formed by coating the foregoing coating liquid (for example, Patent Documents 2 and 3). In the art disclosed in the Patent Documents, however, lowering in the mechanical strength of the foamed particle-containing layer is caused by the constituting materials containing the foam and by the production process thereof. As a result, sticking tends to occur when the thermal transfer image receiving sheet is separated from the thermal transfer ink sheet.
In order to overcome the problem, a multi-layered thermal transfer image receiving sheet having a separation layer on the image receiving layer has been proposed, however, a quality sufficiently acceptable for the recent high speed printing has not been fully attained, and further improvement is demanded.
Patent Document 1: Japanese Patent Publication Open to Public Inspection (hereafter referred to as JP-A) No. 11-301124,
Patent Document 2: JP-A No. 2000-158831
Patent Document 3: JP-A No. 2000-238440